From the previous thread: Here's an explanation of the -51C dewpoint in the RECCO:

97779 23074 51250 91100 88400 99005 78//8 /6973
RMK AF300 0604A DENNIS OB 11
DEW POINT NEG 51C.

In re: to the 78//8 group:
TTTdTdw
TT is the temperature in Celsius. If the temperature is negative, 50 is added to the absolute value of the temperature, and any hundreds digits are omitted. For example, a temperature of -12 would be coded as 62 (12+50 = 62) To determine if a temperature of "10" is really +10 or -110, see the indicator number in the time group above. Missing temperatures are coded as //.
TdTd is the dewpoint at flight level. Dewpoints are encoded the same as temperature. When the dewpoint is colder than -49.4C, it is reported as // and a plain language remark is added with the actual dewpoint, i.e. DEW POINT M53C. // is also reported if the dewpoint is too dry to measure accurately, or missing.